Tyre-changing machines for this type of application generally comprise a rotating chuck provided with a self-centring group for blocking the wheel, which rotating chuck is installed on board a first sliding carriage provided with possibility of nearing and distancing motion in relation to a second sliding carriage, which bears an operating turret provided with a work tool for acting on the wheel for performing the stages of debeading, dismounting and a subsequent mounting of the tyre on the rim.
In particular, the rotating chuck normally comprises a central support element and a plurality of mobile arms distributed about the rotation axis of the chuck, which mobile arms are hinged to the central support element according to hinge axes which lie in a perpendicular plane to the rotation axis of the chuck, and which are singly provided with a respective blocking jaw destined to remain in contact with the rim of the wheel, to block the wheel solidly.
The blocking jaws are distributed along a same circumference which is centred on the rotation axis of the chuck and are substantially step-shaped, such as to make available coupling surfaces for pressing from the inside towards the outside against the channel of the rim, or to grip the edge of the disc of the rim in order to solidly constrain the wheel to the rotating chuck.
The rotating chuck is further provided with means for moving which simultaneously open or lock the mobile arms, such as to distance or alternatively to near the blocking jaws in a radial direction, and means for maintaining the inclination of each blocking jaw constant with respect with the rotation axis of the chuck, during the radial displacements.
In this way, it is possible effectively to vary the diameter of the circumference on which the blocking jaws are distributed, both in order to enable gripping and releasing the rim and to enable the chuck to operate with rims of different diameters.
However, the above-delineated chucks are sometimes not generally able to operate with rims of very large dimensions, for example with rims of a diameter of above 52 inches, typical of tractor wheels, such that the largest opening of the mobile arms is not however sufficient for the blocking jaws to couple with the internal surfaces of the rims.
In order to block rims of these sizes, at present use is made of prolongating elements which are mounted manually on each blocking jaw by the operator, and which are designed to increase the overall radial development of the blocking jaw, such as to reach the internal surfaces of even the largest rims.
The prolongation elements are however separate accessories from the tyre-changing machine, which are used only occasionally and which thus can easily be lost or damaged. Further, the mounting and successive dismounting operations of these accessories from the blocking jaws considerably increase the equipping and preparation times of the tyre-changing machines, as well as the work and the effort required of the operators.
The aim of the present invention is to overcome the above-mentioned drawbacks in the prior art, in the ambit of a simple, rational and inexpensive solution.
The aim is attained by a tyre-changing machine having the characteristics reported in independent claim 1. The dependent claims delineate preferred or particularly advantageous aspects of the invention.